docs: def graph_png(self):
          """
          Export a graph of the data in png format using graphviz/dot.
          """
          if not self.out_file:
              ui.error(c.MESSAGES["png_missing_out"])
              sys.exit(1)
  
          cli_flags = "-Gsize='{0}' -Gdpi='{1}' {2} ".format(self.size, self.dpi,
                                                             self.flags)
          cli_flags += "-o {0}".format(self.out_file)
  
          (out, err) = utils.capture_shell(
              "ansigenome export -t graph -f dot | dot -Tpng {0}"
              .format(cli_flags))
  
          if err:
              ui.error(err)